text: The BRET is the creation of Professor J R Cameron. The BRET value corresponding to a dose of radiation is the number of days of average natural background dose it is equivalent to. It is calculated from the equivalent dose in sieverts by dividing by the average annual background radiation dose in Sv, and multiplying by 365:
  The definition of the BRET unit is apparently unstandardized, and depends on what value is used for the average annual background radiation dose, which varies greatly across time and location. The 2000 UNSCEAR estimate for worldwide average natural background radiation dose is 2.4 mSv (240 mrem), with a range from 1 to 13 mSv. A small area in India as high as 30 mSv (3 rem). Using the 2.4 mSv value each BRET unit equals 6.6 μSv.
  BRET values for diagnostic radiography procedures range from 2 BRET for a dental x-ray to around 400 for a barium enema study.